1. Field of the Invention
The present invention relates to nonreciprocal circuit devices, and more particularly, it relates to nonreciprocal circuit devices such as isolators and circulators used in microwave bands, and communication apparatuses incorporating the nonreciprocal circuit devices.
2. Description of the Related Art
In general, a lumped-constant isolator adopted in a mobile communication apparatus such as a mobile phone passes a signal only in a direction in which the signal is transmitted, while blocking the signal transmission in the opposing direction. In addition, in the recent mobile communication apparatus, compact, lightweight, and low-priced products have been strongly demanded. With this tendency, a compact, lightweight, and low-priced isolator has been demanded.
As the lumped-constant isolator, there is known a device such as a lump-constant isolator 11 shown in FIG. 13. In the lump-constant isolator 11, a resin terminal case 13 is disposed on a metal lower case unit 12 having left and right walls 12a and a bottom wall 12b. A central electrode assembly 14 is contained in the terminal case 13, and a metal upper case unit 15 is disposed on the structure. A permanent magnet 16 is attached to the inner surface of the metal upper case unit 15. With the permanent magnet 16, a direct current magnetic field is applied to the central electrode assembly 14.
The central electrode assembly 14 is arranged by crossing three central electrodes 21 to 23 electrically insulated from each other at angles of 120 degrees on the upper surface of a microwave ferrite member 20. Ports P1 to P3 of ends of the three central electrodes 21 to 23 are horizontally extracted, and a common shield part of the other ends thereof is in contact with the lower surface of the ferrite member 20. The common shield part, which substantially covers the lower surface thereof, is connected to the bottom wall 12b of the lower case unit 12 via a window 13a of the terminal case 13.
The ports P1 to P3 of the central electrodes 21 to 23 are connected to the hot-side capacitor electrodes of matching capacitors C1 to C3. An end of a terminating resistor R is connected to the hot-side capacitor electrode of the matching capacitors C3. The central electrode assembly 14 and the capacitors C1 to C3 are contained in the terminal case 13. Then, as shown in FIG. 14, the edges of two parts where the upper case unit 15 and the lower case unit 12 are close to each other (areas shown by vertical lines in FIG. 14) are connected by solder 18 (see FIG. 15).
Meanwhile, the conventional isolator 11 forms a frame-like loop structure around the permanent magnet 16 and the central electrode assembly 14 in the upper case unit 15 and the lower case unit 12. Thus, as shown in FIG. 15, a high frequency current i is likely to go around the upper and lower case units 15 and 12. As a result, there is a problem of power consumption due to Joule loss. Moreover, the high frequency current i flowing through the case units 15 and 12 serves to cancel a regular signal current I flowing through the central electrodes 21 to 23. In other words, the high frequency current i serves to reduce a high frequency magnetic field generated by the ferrite member 20. As a result, the effective inductances of the central electrodes 21 to 23 and the effective magnetic permeability of the ferrite member 20 are reduced, thereby narrowing the operational frequency bandwidth of the isolator 11.
Accordingly, it is an object of the present invention to provide a nonreciprocal circuit device including a metal case through which a high frequency current is difficult to flow. In addition, it is another object of the present invention to provide a communication apparatus incorporating the nonreciprocal circuit device.
In order to accomplish the above objects, the present invention provides a nonreciprocal circuit device including a permanent magnet, a ferrite member which is adapted to receive a direct current magnetic field applied by the permanent magnet, the ferrite member including a plurality of central electrodes, and a metal case containing the permanent magnet, the ferrite member, and the plurality of central electrodes. In this nonreciprocal circuit device, the metal case has a gap for cutting off a loop current flowing around the ferrite member and the plurality of central electrodes.
In this case, a xe2x80x9cgapxe2x80x9d means an electrical gap. The gap of the present invention also includes a gap in which an insulating material is filled. In such a situation, although there is physically no gap, there is no electrical connection.
For example, the cross section of the metal case may have a substantially rectangular frame or cylindrical shape by bending a substantially rectangular metal plate at four positions in parallel to an edge of the metal plate. In addition, the metal case may be constituted of an upper case unit and a lower case unit. There may be disposed a gap at least between one edge of the upper case unit and an edge of the lower case unit opposite to the edge of the upper case unit. Furthermore, preferably, the metal case is set to be rotation-symmetrical with respect to the axis of the permanent magnet.
With the above arrangement, since a high frequency current flowing through the metal case is cut off by the gap disposed in the metal case, the high frequency current is difficult to flow through the metal case.
In addition, since the communication apparatus incorporating the above nonreciprocal circuit device in accordance with the present invention can have good frequency characteristics.